This invention relates to a new and improved circuit for detecting temperature changes such as overheating in a line wire, changes in skin temperature of an individual, etc. The circuit is particularly useful in detecting a temperature increase over that portion of a thermistor detection curve which is changing slowly in the area of interest, i.e., about 300.degree. K.-400.degree. K. By adapting the circuit to detect these relatively small changes, it is easier to provide a warning before the thermistor detection curve begins to move in a steep upward incline where it is difficult to sound an alarm at a specific point.
Widely differing types of circuit overloading or an overheating condition can be detected quickly and accurately. One type of an overheated condition is produced by the overloading of an extension cord or electrical device. It would be preferred to detect the onset of this overheated condition rather than wait for a marked temperature rise or runaway overheating before sounding an alarm. Also, many extension cords of say, 18 gauge have a capacity of about 5-10 amps and they can burn out before a circuit breaker operating at 15 amps will detect this condition. Furthermore, for each 10.degree. C. rise in temperature, a doubling in chemical reaction rates, such as in a line wire burn, will occur. Hence, even if a circuit breaker reacts in time, considerable energy loss and line wire decomposition can occur.
Another type of an overheated condition is caused by failure of a person undergoing physical activity to perspire adequately when performing physical exercise such as calisthenics, cycling, swimming, jogging, etc. Failure to perspire effectively might indicate a person's heat dissipating mechanism has become impaired, thereby causing an internal heat build up, or the individual has become dehydrated, etc. This condition can be accompanied by a decrease of evaporative losses and a sudden rise in skin temperature which reflects central body temperature. If the individual were made aware of this sudden temperature rise, activity could be ceased before a potentially dangerous body condition became aggravated; also, remedial steps could be taken, such as lowering the activity rate, resting, cooling off, taking oxygen, drinking fluids, etc. In the case of a patient, a sudden temperature increase might indicate some type of emergency treatment was necessary.
In certain instances, the heartbeat of a person undergoing exercise may accelerate, and this would alert the individual to slow down activity. However, in other cases, heart stroke volume increases, and this can pose a danger because the person undergoing exercise does not realize the heart function has increased, and the exercise is continued rather than being slowed down. Examples of heat intolerant individuals are reported in "Body Fluid Response of Heat-Tolerant and Intolerant Men To Work In a Hot Wet Environment", Journal of Applied Physiology, Vol. 40, No. 1, Jan. 1976, by L. C. Senay and R. Kok.